virtual particles

The law of conservation of mass and energy, so imperious and omnipresent, refuses to act with full severity during negligibly short periods of time. The sun and the planets with lifetimes of thousands of millions of years, on the one hand, atoms and particles with lifetimes of millionths of a second, on the other hand, obey the law with equal submissiveness. Quite another matter for particles whose lifetime is so short that in each specific case it cannot be noted at all. The conservation law does not condescend to follow their destiny, to see that they obey the laws of behaviour, which are, it would seem, accepted equally in the mega-, macro- and microscopic worlds. One physicist remarked, in this connection, that a particle in the given situation, behaves like a swindling cashier, who regularly manages to return money taken from the cashbox before anybody notices the loss. A particle is created out of "nothing" and disappears on the spot. "On the spot" for such an instantaneous neutron means a lifetime of about 10-24 s. An instant electron is lighter, i.e, its mass is less by a factor of two thousand and it can exist two thousand times longer than a neutron, or approximately 10-21 s. An ordinary free neutron has a lifetime of several minutes, and when it is part of an atomic nucleus, it can live indefinitely long, as indefinitely long as an electron if you leave it alone. The conservation laws, as you can see, do not for long leave their violaters in peace.

In contrast to ordinary particles, these ephemeral ones are said to be virtual. In this context, they are possible particles. This opens for philosophers a field of application to the concrete picture of a physical vacuum, ancient discussions on just in what the possible differs from the actual, etc.

Nevertheless, the meaning of the name itself should in no way imply that the given particles are possible, since they are called virtual, but actually they do not exist. It should be clear that that which does not exist cannot affect anything, whatever it is. But the possible particles in a vacuum have a quite real effect, as can be observed in precise experiments, on quite real formations of undoubtedly real elementary particles and even on macroscopic bodies.

In what, besides their negligibly short lifetime, do virtual particles differ from their twins in the real world? These "violaters" of the law of conservation of mass and energy do not have the ordinary relation between energy, momentum and mass. But, to make up for it, all their remaining characteristics are fully respectahle. An electron remains an electron in the virtual state as well, a proton remains a proton, etc. They retain their charges and other typical properties with enviable constancy, but only for a very short time, between their creation and disappearance.

The ephemerality of virtual particles leads to a situation in which it is absolutely impossible, according to up-to-date conceptions, to discover such particles experimentally, to register them in some way. They leave no traces in physical instruments.

So what are we to do? Will their existence, following from mathematical calculations, remain a purely paper phenomenon in which you are free to believe or not believe, a phenomenon that may disappear when a change is made in the theory?

Physicists believe that this is not so. Besides mathematics, they have in their arsenal the finest of experimental techniques. Whereas they cannot detect separate virtual particles of vacuum, their total effect on ordinary particles can be registered experimentally.

Let us consider the hydrogen atom. Its nucleus is a single proton about which a single electron travels. (Today the electron on its orbit is no longer conceived of as something like a solid sphere, it is rather more like a cloud smeared at a definite distance from the nucleus along the whole orbit.)

The effect of virtual particles compels the electron to deviate chaotically first to one side and then to the other or the path along which it would travel if there were no virtual particles at all. This phenomenon is called vacuum tremor of the electron. The hydrogen electron is an entirely real particle, faithfully obeying the law of conservation of mass and energy. Hence, the oscillation of the electron on its orbit leads to a change in its potential energy. Such a change can be reliably registered. The phenomenon is called the Lamb shift in honour of the American physicist Willis Eugene Lamb, Jr., who together with his compatriot Robert Retherford first discovered such a shift in 1947.

The magnetic moment of the electron (it characterizes the interaction between a particle and the external magnetic field) also, by its magnitude, bears witness to traces of the effect of vacuum virtual particles. This trace is so clear that the experimentally determined magnetic moment of the electron was previously said to be anomalous, because it differed so greatly from that predicted by theoretical calculations. Now the attribute "anomalous" is purely historical. Today, calculations on the basis of quantum field theory with the effect of the vacuum taken into account yield values that coincide excellently with experimental data.

Here is another example. According to Maxwell's theory, photons should not interact with one another. But experimentally, such interaction, however small, has been observed. Again, it is the virtual particles that are to blame.

It was found possible to observe the effect of vacuum virtual particles, not only in experiments for investigating the interactions of elementary particles, but also in experiments involving macroscopic bodies. Two plates, placed into a vacuum and brought close together, begin to attract each other due to the impacts of the virtual particles. This fact was discovered in 1956 by the Dutch theoretical and experimental physicist Hendrik B. G. Casimir and was called, in his honour, the Casimir effect. The fact, is that absolutely all reactions, all interactions between real elementary particles take place with the indispensable participation of a vacuum virtual background, which, in its turn, is also affected by the elementary particles.

It is necessary to point out that according to up-to-date physical concepts, virtual particles appear not only in vacuum. They are also created by ordinary particles. Electrons, for instance, continually emit and immediately absorb virtual photons at such a high rate that the gain in energy during the short lifetime of such a photon cannot, in principle, be observed.

And what is more, any interaction between elementary particles can be dealt with as including the emission and absorption of virtual particles, as an exchange of them.

Furthermore, a real electron attracts virtual positrons and repulses virtual electrons, in accordance with the law we learned in school: electric charges of like kind repel each other, while charges of unlike kind attract each other. As a result the vacuum is polarized because the charges in it are separated in space. An electron surrounded by a layer of virtual positrons turns out to be behind a real screen of such particles. This reduces the so-called effective charge of the electron manifested in its interactions with other particles. The polarization of vacuum, as will be evident further on, is a process that should play an exceptionally important role in many physical events.

Each elementary particle, physicists believe today, travels in company with a whole retinue of virtual particles. Dmitri Ivanovich Blokhintsev, associate member of the USSR Academy of Sciences, wrote: "... As a result of the polarization of a vacuum, a charged 'atmosphere' is set up around a charged particle and is linked to this particle".

More often lately, in the Soviet literature, the cloud of virtual particles about a particle is called a coat. A coat can consist of several layers; it pulsates: sometimes appearing, sometimes disappearing and leaving its bearer "bare". Is it possible to disregard such a garment?

Now, when we are dealing with an elementary particle without its indispensable virtual companions, capable of so drastically altering some of its properties, physicists speak of its bare mass and its bare charge, and they admit that the properties are poorly determined. The virtual cortege prevents the distinguished person they are escorting from being properly viewed.

In connection with the problems facing physics, Niels Bohr contended that a person in our time devotes himself to problems that take his breath away and turn his head, but, you feel slightly giddy, you cannot understand their essence. Bohr continued by stating that problems are more important than their solution; solutions may become obsolete, but the problems remain. [Something Called Nothing, pages 75-79]

See Also

13 - Chart Defining the Arrangement of the different Atoms and Corpuscles of Matter
Alpha Particle
beta particle
corpuscular bombardment
corpuscular confliction
electrical particle
electrically charged particles
flux of virtual photons
God Particle
K particle
light as wave and particle
No particles in nucleus - Russell
particle of matter
particle physics
Particles and Corpuscles
shimmer of motes
standard model elementary particles
Table of Quantum Particles
ultimate particle of Creation
virtual particle flux
virtual particle flux of vacuum
virtual photon flux
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